The present invention is concerned with a process for preparing acetone by catalytic oxidative decarbonylation of isobutyraldehyde in the gaseous phase, which evidences extraordinarily high selectivity at full conversion of isobutyraldehyde.
Isobutyraldehyde is obtained in large amounts as an inevitable by-product when propene is hydroformylated to n-butyraldehyde. The isobutyraldehyde by-product has failed to be applicable in any way to covering the costs necessary for the propene starting material and the hydroformylation process. There is a technical possibility of efficiently processing the isobutyraldehyde by catalytically oxidizing it to acetone. The two processes known are the liquid and the gaseous phase processes.
Higher yields and better selectivities are obtained when oxidizing in the gaseous phase as opposed to the liquid phase. In this process, gaseous isobutyraldehyde is passed together with molecular oxygen in the presence of an inert diluent over a carrier catalyst.
The state of the art of preparing acetone by the catalytic oxidative decarbonylation of isobutyraldehyde in the gaseous phase may be ascertained by reference to U.S. Pat. Nos. 3,804,902; 3,855,304 and 4,000,199, the disclosures of which are incorporated herein.
German Published application No. 21 57 307 and U.S. Pat. Nos. 3,804,902 and 3,855,304 disclose a catalyst of manganese oxide on activated aluminum oxide. While such a catalyst offers high yields, the selectivities on the other hand are relatively slight. Better selectivities are obtained using copper oxide on activated aluminum oxide or zinc oxide, as disclosed in U.S. Pat. No. 4,000,199. This process too suffers from the drawback that a still appreciable proportion of the isobutyraldehyde is burned to carbon dioxide. Thus, under the most favorable conditions, using copper oxide on aluminum oxide, a selectivity of 93 mol % is obtained, i.e., from 6 to 7 mol % of isobutyraldehyde are burned to carbon dioxide and water. When copper oxide on zinc oxide is used, a selectivity of 96 mol % is in fact obtained, but the yield on the other hand is only 90.5%. The economy of such a process of the present invention, however, demands high yield and high selectivity, because basically valuable propene, which is contained in the isobutyraldehyde, is burned to useless carbon dioxide and water in the total combustion of the isobutyraldehyde. In addition, about fivefold the amount of the heat released per mole of isobutyraldehyde in the oxidative decarbonylation of the isobutyraldehyde to acetone is released in the total combustion. Even when there is only 6 mol % of total combustion, the ratio of the amounts of heat of total to partial combustion is about 1:3. These additional large amounts of heat magnify the problem of heat transfer and entail a lower throughput over the catalyst, whereby again the economy of the process is decreased.